Why can we use `nil` to get members-CodePudding

I am studying http source codes of Golang, I found this

func NewRequestWithContext(ctx context.Context, method, url string, body io.Reader) (*Request, error) {
    ...
    rc, ok := body.(io.ReadCloser)
    ...
}

but this body is nil, it is passed in the following way

func NewRequest(method, url string, body io.Reader) (*Request, error) {
    return NewRequestWithContext(context.Background(), method, url, body)
}
func (c *Client) Get(url string) (resp *Response, err error) {
    req, err := NewRequest("GET", url, nil)
    if err != nil {
        return nil, err
    }
    return c.Do(req)
}

Function Get passes nil to function NewRequest,

function NewRequest passes this nil to function NewRequestWithContext,

then function NewRequestWithContext use nil to call nil.(io.ReadCloser), why does not it cause panic?

CodePudding user response：

The statement

    rc, ok := body.(io.ReadCloser)

tests if body is a ReadCloser. If not, then rc will be set to nil, and ok will be set to false.

If the code was:

rc:=body.(io.ReadCloser)

then with a nil body, it would panic.

CodePudding user response：

The statement rc, ok := body.(io.ReadCloser) is a type assertion used in the assignment special form. When body is nil, rc is set to nil and ok is set to false.